There are a number of toner resin characteristics which are influenced by molecular weight and architecture. For example, the molecular weight and degree of branching of the toner resin directly affect melt viscosity, melt elasticity, toner keeping, photoconductor scumming and grinding characteristics. Therefore, it is desirable to control the molecular weight and molecular architecture of electrophotographic binder polymers quite precisely. One of the most effective ways to achieve this control over molecular weight and chain architecture is through the well-known use of mercaptan-type (RSH) chain transfer agents which are added directly to the monomer mixture prior to initiation of the polymerization reaction. These types of chain transfer agents possess high transfer coefficients for styrene-based systems such as commonly used in the electrophotographic industry. Thus, they provide excellent control over polymer molecular weight and branching when they are used in conjunction with a crosslinking agent such as divinylbenzene.
Additionally, these RSH-type chain transfer agents are quite effective when used with azo-type initiator systems which are the preferred initiators for suspension polymerizations. However, the use of RSH-type chain transfer agents creates a significant problem. The mercaptan groups are incorporated into the polymer chains mainly as sulfide (RS-P, where P=the polymer chain) end groups as a necessary consequence of their chain-transfer activity (see Reaction 1 below). EQU RSH+P.fwdarw.RS.sup..multidot. +PH RS.sup..multidot. +M.fwdarw.RSM.sup..multidot. (new chain) (M=monomer) (1)
As long as these sulfur species are incorporated into polymer chains their vapor pressure is low. However, in toner manufacture the binder polymers are subjected to high temperatures and shear resulting in degradation of the polymer molecular weight. This phenomenon can lead to the regeneration of free thiol compounds from the chain ends as well as other thiol-containing species which may be present in much lower molecular weight fractions than originally envisioned. Some of these thermally generated species are low enough in molecular weight to possess appreciable vapor pressures, particularly when present in high heat areas of copiers such as the fusing station. Since these thiols have strong, objectionable odors typical of most sulfur compounds their volatilization presents an environmental problem for the users of toners containing these impurities. These sulfur compounds can have odor thresholds as low as two parts per billion. Additionally, these thermally generated thiol compounds can further be oxidatively degraded to yield sulfur dioxide (SO.sub.2) a highly volatile, malodorous gas which can cause severe respiratory distress. Thus, in order to produce a low odor toner with molecular architecture control through mercaptan-mediated chain transfer it is necessary to minimize the presence of the low molecular weight mercaptans as well as their oxidation products. Since odor thresholds are so low for these compounds and the compounds are released or formed upon heating it is difficult, if not impossible to achieve odor-free polymer by conventional purification methods, that is by washing, slurrying, etc.
One approach to reduce odors is to use a high molecular weight mercaptan or to make the molecular weight of the mercaptan higher by reacting it with substances that do not effect its molecular weight modifying character. This approach prevents odors by making the mercaptan less volatile. Examples of this approach are found in JP 92 161,403 where odorless latexes were produced using t-dodecanethiol as the chain transfer agent; JP 92 93,308 in which odorless rubber modified styrene polymers were produced using t-dodecanethiol; U.S. Pat. No. 4,450,261 in which low molecular weight styrene maleic anhydride copolymers were produced using 3-mercaptopropionic acid or methyl ester of 3-mercaptopropionic acid as the chain transfer agent; DE 2,525,413 in which t-butyl mercaptan is used in the grafting step during preparation of ABS resins and found to give lower odors than either propyl or isopropyl mercaptan; JP 75 32,281 in which low odor acrylonitrile-butadiene copolymers using n-dodecyl mercaptan in combination with 1,4-cyclohexadiene; and finally JP 73 96,832 in which odorless acrylic fibers were obtained using thioglycerol as the chain transfer agent during synthesis.
Another approach is to react the mercaptan and convert it to an odor-free moiety. Examples of this approach are found in JP 93 247,893; JP 91 277,602; JP 91 207,367; EP 147,949; DE 3,546,680 and U.S. Pat. No. 3,374,287.
A third approach to remove residual mercaptans and low molecular weight mercaptan adducts is by post process distillation as in DE 2,611,222.
The above cited methods all teach methods of reducing odors due to mercaptans. However, none of these are applicable to the reduction of mercaptan odor in polymers synthesized for use as toner binders. Therefore, there is a need for an invention which reduces the odor occurring in toner binders.
The present invention solves the above-identified problem by synthesizing mercaptan odor-free polymers for use as toner binders.